The Omagh Support and Self Help Group report focuses on the role of two state agents who infiltrated the Real IRA. Photograph: Mike Mahoney/Reuters

MI5, the FBI and Garda special branch "starved" police in Northern Ireland of vital intelligence that could have prevented the Real IRA bomb that killed 29 people at Omagh in 1998, a damning new report on the atrocity concludes.

The investigation, commissioned by families of the Omagh victims, will show evidence that they claim proves that information from two key informers inside the Real IRA – one in the United States, the other in the Irish Republic – was not passed on to the Royal Ulster Constabulary.

The Omagh bomb was the single biggest atrocity of the Northern Ireland Troubles. No one has been convicted in a criminal court in connection with the bombing in the County Tyrone market town.

Ahead of the publication of the report in Omagh, the father of Aidan Gallagher, a young man killed in the blast, said access to new intelligence files proved that the FBI, the security services and the Garda's crime and security branch (the Republic of Ireland's main anti-terrorist unit) all withheld vital information.

Michael Gallagher, who has campaigned since the atrocity for a cross-border public inquiry, said: "All good policing is based on intelligence, especially prior intelligence before any criminal act is committed. In the case of the events running up to the Omagh bomb, it is now clear that the police in the north were starved of information. The security forces in America, Britain and the Republic had two key agents inside the Real IRA but did not share the information they were providing to the police in Northern Ireland."

"There has been no full investigation into the circumstances surrounding the Omagh bomb. The inquest did not inquire into the intelligence, the criminal prosecutions did not lead to any convictions, and the civil action did not deal with the issue of preventability. The police investigation has been heavily criticised and the report highlights such concerns that the states [UK and Ireland] must now establish a full cross-border public inquiry," a spokesperson for the group said, adding that failure to do so would be a failure to comply with obligations under article 2(1) and article 3 of the European convention on human rights.

"This report is compiled from a significant amount of information some of a sensitive nature which has come into the possession of the families," the spokesperson said.

The report, compiled by London law firm SBP, aided by several retired security experts, focuses on the role of two state agents who infiltrated the Real IRA. They were David Rupert, an American who was run by the FBI, and Paddy Dixon, a convicted criminal who procured cars in the Irish Republic for the Real IRA for transporting car bombs into Northern Ireland throughout 1998.

Rupert arrived in Ireland in the mid 1990s and offered his services first to the Continuity IRA and later the Real IRA. The 6ft 5in, 20-stone bankrupt businessman with links to Irish Americans had been targeted by the FBI as a potential agent.

By the summer of 1997, Rupert became involved with British intelligence. An FBI agent took him to a hotel in central London, where he was introduced to an MI5 officer who called himself Norman.

Norman advised Rupert not to pass all his information to the Gardai and provided him with a PO box address and a secret contact phone number.

MI5 urged him to offer intelligence to the Continuity IRA about British army and police bases in Northern Ireland. Posing as American tourists awe-struck by the security installations, Rupert and his wife would stop at border crossing points such as Aughnacloy and take pictures and videos of themselves. According to the Omagh Self Help and Support group new intelligence files indicate Rupert was also aiding the Real IRA, including sending bomb component parts from America. It was Rupert's testimony that helped convict the Real IRA founder Mickey McKevitt of organising acts of terrorism in 2003.

Dixon provided intelligence to a Garda handler on the Real IRA throughout 1998. He had a long-standing connection with a republican in south Co Dublin known as the Long Fellow. His handler suggested – under orders from senior Garda command – that his old agent reactivate his relationship with the Long Fellow, who owned a breaker's yard in south Dublin where Dixon's stolen cars were replaced and huge explosive devices were secreted inside the vehicles.

Over the next seven months Dixon would give the Garda vital insight into the Real IRA terror machine. Between February and August 1998 Dixon gave the Irish police force inside information on at least nine separate Real IRA attacks culminating in the bomb at Omagh.

The Omagh families claim the "starving of intelligence" coming from Rupert and Dixon was designed to bolster the pair's credibility in the Real IRA's eyes. However, the Omagh campaigners claim the new report will show that this was a lethal error of judgment in relation to the terror group's final and most catastrophic attack in 1998.